High strength bake hardenable low alloy steel and process for manufacture thereof

ABSTRACT

A bake hardenable steel and a process for manufacture of a bake hardenable steel. Steel alloy slabs with a predefined chemical composition are hot rolled into hot rolled strip. The hot rolled strip is cold rolled and continuously annealed on a continuous annealing line and then rapidly cooled to at least 600° C. using a cooling rate of between 20-100 K/sec, and optionally cooled to at least 140° C. using a cooling rate of between 3-20 K/sec. As such, the cold rolled and annealed sheet is not subjected to an overageing treatment and yet still exhibits excellent and well defined bake hardening values. For example BH2 values between 25 and 45 MPa are exhibited by the low alloy steel grades. In addition, the bake hardeneable steel sheet has a minimum yield strength of 220 MPa and in some instances has a minimum yield strength as high as 380 MPa.Zone

RELATED APPLICATION

The instant application claims priority of U.S. Provisional Application No. 61/793,444 filed on Mar. 15, 2013, the contents of which are incorporated herein in their entirety by reference.

FIELD OF THE INVENTION

The present invention is related to a high strength alloy steel, and in particular, to a high strength bake hardenable low alloy steel.

BACKGROUND OF THE INVENTION

The use of high strength steel for automotive components is known. In addition, the use of such steels to reduce vehicle weight and improve gas mileage economy is also known. However, due to their relatively high strength, sheet of the material tends not to conform closely to stamping dies during stamping or forming and thus causes undesirable surface deflection or splits/cracks in the formed parts. In response to this problem, steel manufacturers have developed steel that has a relatively low yield strength before stamping and then a higher yield strength in a final finished product.

One class of such steels is bake hardenable (BH) steels (also known as bake hardening steels) that allow for hardening or strengthening during a paint baking or coating treatment. Conventional BH steel grades for automotive applications are continually casted and fully killed by aluminum. Nitrogen within the steel precipitates as aluminum nitride and contributes to grain size control. In addition, any residual nitrogen is captured by either titanium, boron, vanadium, and/or niobium and thus plays no role in the bake hardening effect of the material. As such, the bake hardening effect results from free carbon alone, i.e. carbon that is in solid solution before the paint baking or coating treatment.

The paint baking or coating treatment typically occurs for about 20-40 minutes at temperatures between 120-250° C. However, heretofore BH steels have required thermomechanical processing that includes an over-aging treatment. The over-aging treatment typically occurs after a post cold rolling annealing treatment and rapid cooling. In continuous annealing lines coils are heated to the annealing temperature and soaked in furnaces, and quenched in rapid cooling section after passing through gas jet cooling section. Coils are then subjected to over aging treatment in an overaging section. In the overaging section the cooling of the steel sheet is interrupted and held within a lower temperature range such as 350-450° C. for a predetermined period of time in order to precipitate carbides from excess free carbon.

After receiving over aging treatment in over aging section, the coils are temper-rolled so that their surface roughness is finished and shapes conditioned.

Naturally, such an over-aging treatment consumes additional energy compared to a continuous cooling of the material. As such, a low alloy BH steel and/or a process of manufacture for a bake hardenable steel that does not require an over-aging treatment would reduce energy costs and be desirable.

SUMMARY OF THE INVENTION

A bake hardenable (BH) steel and a process for manufacture of a BH steel is provided. The BH steel and/or process disclosed herein provides for a low alloy steel having a minimum yield strength of 220 megapascals (MPa). In some instances, the inventive low alloy steel has a minimum yield strength of up to 380 MPa. In addition, different compositions and/or processing can be selected such that minimum yield strengths of 240, 260, 280, 300, 320, 340, or 360 MPa are provided.

Compositions of the low alloy steel, in weight percent (wt %), fall within a range of 0.020-0.080 carbon (C), 0.20-1.6 manganese (Mn), 0.0-1.50 silicon (Si), 0.0-0.08 phosphorus (P), 0.0-0.60 chromium (Cr), 0.0-0.40 molybdenum (Mo), 0.0-0.50 copper (Cu), 0.0-0.30 nickel (Ni), 0.04-0.5 aluminum (Al), and the remainder being iron (Fe) and incidental impurities known to those skilled in the art. In addition, titanium (Ti) up to 0.025 and/or boron (B) up to 0.0070 can be utilized to combine with nitrogen (N).

Steel slabs having a chemical composition within the above stated range are hot rolled to produce hot rolled strip which is coiled at temperatures above 600° C. In some instances, the steel slab has a Mn content of less than 0.40, a Si content of less than 0.1, a P content of less than 0.03 and the hot rolled strip made therefrom is coiled at temperatures above 650° C.

The coiled hot rolled strip is cold rolled and continuously annealed on a continuous annealing line (CAL) with an annealing temperature (T_(anneal)) of at least 760° C. (T_(anneal)≧760° C.), and in some instances with an annealing temperature of at least 780° C. (T_(anneal)≧780° C.). In some instances, the annealed cold rolled sheet is rapidly cooled to at least 600° C. using a cooling rate of between 20-100 K/sec. In other instances, the annealed cold rolled sheet is ultra-rapidly cooled to at least 375° C. using a cooling rate of between 50-120 K/sec and optionally cooled to at least 140° C. using a cooling rate of between 3-20 K/sec before exiting the CAL furnace. Also, the time spent in the cooling section of the furnace (calculated from the exit of the rapid cooling section to the furnace exit) is less than 4 minutes. In some instances, slow cooling from the annealing temperature to 720° C. ±20° C. using cooling rates between 2-10 K/s can be applied after the annealing treatment and before the rapid cooling. Also, the cold rolled and annealed sheet may or may not be subjected to a hot dip coating process, e.g. hot dip galvanizing or hot dip aluminizing.

The inventive steels and/or processing afford for BH2 values greater than or equal to 20 and less than 60 MPa without the use or need of overaging treatments. In addition, the C and Mn contents can be controlled such that specific combinations of C and Mn provide cold rolled and annealed sheet with BH2 values between 25-45 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical plot illustrating a desired range of C and Mn (Zone (A)) for alloys according to embodiments of the present invention;

FIG. 2 is a graphical representation of a prior art heat cycle for a prior art BH steel grade;

FIG. 3 is a graphical illustration of an inventive heat cycle for BH steel grades according to an embodiment of the present invention;

FIG. 4 is a graphical illustration of an inventive heat cycle for a galvanized BH steel grades according to an embodiment of the present invention;

FIG. 5 is a graphical plot of yield strength (0.2% offset) versus position along the length of inventive coils—head (H), middle (M), tail (T)—for alloys according to an embodiment of the present invention;

FIG. 6 is a graphical plot of yield strength (lower yield strength) versus position along the length of inventive coils—head (H), middle (M), tail (T)—for alloys according to an embodiment of the present invention;

FIG. 7 is a graphical plot for ultimate tensile strength versus position along the length of inventive coils—head (H), middle (M), tail (T)—for alloys according to an embodiment of the present invention;

FIG. 8 is a graphical plot for percent elongation to fracture versus position along the length of inventive coils—head (H), middle (M), tail (T)—for alloys according to an embodiment of the present invention;

FIG. 9 is a graphical plot of n-value versus position along the length of inventive coils—head (H), middle (M), tail (T)—for alloys according to an embodiment of the present invention;

FIG. 10 is a graphical plot of R-bar versus position along the length of inventive coils—head (H), middle (M), tail (T)—for alloys according to an embodiment of the present invention;

FIG. 11 is a graphical plot of Bake Hard Index Values versus position along the length of inventive coils—head (H), middle (M), tail (T)—for alloys according to an embodiment of the present invention;

FIG. 12 is a series of optical micrographs of the longitudinal direction microstructure etched with 2% nital for inventive 220-300 YS (MPa) steel coils taken at magnifications of: (A) 200×; (B) 500×; and (C) 1000×;

FIG. 13 is a series of optical micrographs of the transverse direction microstructure etched with 2% nital for inventive 220-300 YS (MPa) steel coils taken at magnifications of: (A) 200×; (B) 500×; and (C) 1000×;

FIG. 14 is a series of optical micrographs of the longitudinal direction microstructure etched with 2% nital for inventive 280-340 YS (MPa) steel coils taken at magnifications of: (A) 200×; (B) 500×; and (C) 1000×;

FIG. 15 is a series of optical micrographs of the transverse direction microstructure etched with 2% nital for inventive 280-340 YS (MPa) steel coils taken at magnifications of: (A) 200×; (B) 500×; and (C) 1000×;

FIG. 16 is a series of optical micrographs the longitudinal direction microstructure etched with 2% nital for inventive 300-380 YS (MPa) steel coils taken at magnifications of: (A) 200×; (B) 500×; and (C) 1000×; and

FIG. 17 is a series of optical micrographs of the transverse direction microstructure etched with 2% nital for inventive 300-380 YS (MPa) steel coils taken at magnifications of: (A) 200×; (B) 500×; and (C) 1000×.

DETAILED DESCRIPTION OF THE INVENTION

A bake hardenable (BH) steel and a process for manufacture of a BH steel is provided. As such, the invention has use as a material and/or a process for making a material that can be used to produce automotive components.

The BH steel and/or process disclosed herein provides for a low alloy steel having a minimum yield strength of 220 megapascals (MPa). In some instances, the inventive low alloy steel has a minimal yield strength of up to 380 MPa. In addition, different compositions and/or processing of the different compositions provides for BH steel grades with minimum yield strengths of 240, 260, 280, 300, 320, 340, or 360 MPa.

Compositions (in wt%) of the low alloy steel fall within steel with 0.020-0.080 C, 0.20-1.6 Mn, 0.0-1.50 Si, 0.0-0.08 P, 0.0-0.60 Cr, 0.0-0.40 Mo, 0.0-0.50 Cu, 0.0-0.30 Ni, and 0.04 -0.5 Al, with the remainder or balance being Fe and incidental impurities known to those skilled in the art. In addition, Ti up to 0.025 and/or B up to 0.0070 can be utilized to combine with nitrogen. In addition, BH2 values for the steel are greater than or equal to 20 and less than 60 MPa.

In some instances, the C and Mn contents are restricted to be within 0.02-0.074 and 0.2-1.6, respectively. In addition, the C and Mn contents are controlled such that the inventive steels exhibit BH2 values between 25-45 MPa and obey the relation:

25≦−1300·C+118−{(−1300·C+118)·0.5(Mn−0.2)}≦45   (1)

Such values or chemistries provide a corridor of allowable C and Mn contents as illustratively shown by Zone (A) in FIG. 1.

By controlling C and Mn contents to fall within the corridor shown in FIG. 1, a tight range of desired BH2 values between 25 and 45 MPa are provided without subjecting the cold rolled sheet to an overaging treatment. Not being bound by theory, Mn decreases the C activity and thereby decreases the aging contribution of solute carbon. Mn interacts with interstitial solute atoms C and forms interstitial-substitutional solute pairs. These Mn—C pairs reduce the mobility of the interstitial solutes to interact with dislocations. As such, the mechanism of the instant invention is different from prior art of bake hardenable steels that utilize precipitation mechanisms to control free C concentrations, not the free C activity to control the free C concentration as taught herein.

In addition to the above, free C within the alloy steadily increases with decreasing C content until 0.02 C is reached, i.e. the maximum amount of free C is available at the point of maximum C solubility in ferrite at 723° C. As such, when C content decreases, the Mn content must be increased in order to achieve optimum BH2 values and increasing the Mn content serves the same function as an overaging treatment.

It is appreciated that the term or acronym BH2 refers to the difference between the yield stress measured after a bake hardening treatment and the yield stress after an initial 2% plastic strain. It is also appreciated that a typical bake hardening treatment includes holding the BH steel at temperatures around 170° C. for times ranging from 10-30 minutes.

Turning now to FIG. 2, a typical prior art continuous annealing process for cold rolled and uncoated carbon steel strip is shown. The annealing process includes heating the strip above its recrystallization temperature, e.g. 800° C., soaking of the material at this temperature for a predetermined period of time, followed by an optional slow cooling, and then cooling to an over-aging section or treatment where the material is held between 300-500° C. for 2-3 minutes. After the over-ageing treatment, the strip proceeds to a final cooling section. The over-aging is designed to promote the precipitation of solute carbon dissolved during the annealing steps and to finely control the grain structure for steels with C levels from 0.020-0.045 wt % total C. Such stabilization avoids uncontrolled aging of the material and provides sufficient C in solid solution to enable bake hardening after painting of a component.

An aluminum killed steel with a chemical composition of 0.03% carbon and 0.2% manganese typically delivers a BH grade with a minimum yield strength of 180 MPa. In addition, low alloy steels having the approximate same carbon level, i.e. around 0.03% carbon, can achieve higher minimum yield strengths only by adding elements such as silicon, phosphorus, and manganese.

In contrast, the inventive bake hardenable steel and process allows for the production of bake hardening steel having a minimum yield strength of 220 MPa with a reduction in energy and alloy cost. Furthermore, the bake hardenable steels disclosed herein can be electrocoated for corrosion protection.

Compared to hot dip galvanizing processes for higher carbon content BH grades, the disclosed bake hardenable low alloy steel is not limited by alloy element restrictions when subjected to continuous annealing. In addition, temper rolling of the inventive material can be applied, such temper rolling providing between 1 and 2% elongation to the material.

Referring now to FIG. 3, an inventive heat cycle is shown for a low alloy steel having a chemical composition within the chemistry range discussed above is hot rolled, cold rolled, and then annealed at an elevated temperature such as 800° C. The annealed cold rolled sheet is then rapidly cooled to a temperature equal to or less than 600° C. In some instances, the annealed cold rolled sheet is ultra-rapidly cooled to at least 375° C., and additional but less rapid cooling to temperatures to 140° C. and lower can follow. Total time spent in the furnace cooling section is less than 4 minutes. In addition, slow cooling from the annealing temperature to 720° C. ±20° C. using cooling rates between 2-10 K/s may or may not be applied after the annealing treatment and before rapid cooling. The rapid cooling can be by gas jet cooling that produces a cooling rate between 20 K/s and 120 K/s. The cooling rate of the less rapid cooling treatment down to at least 140° C. can be between 3 K/s and 20 K/s until the strip reaches a furnace exit temperature of 140° C. or lower. Roll quenching in the course of cooling can also be provided and thereby lead to higher cooling rates than described above.

Looking now at FIG. 4, an inventive temperature profile for annealing, followed by cooling, hot dip galvanizing and final cooling is shown. FIG. 4 illustrates, and it should be appreciated, that the process in principle is known for hot dip galvanizing of strip steel but that such a heat cycle has not heretofore been used for production of bake hardening grades on continuous annealing lines. It should also be appreciated that continuous annealing of uncoated strip steel affords for alloy elements such as Si, Mo, Cr and other alloying elements, which typically deteriorate surface quality of galvanized material, to be utilized in the instant invention to increase strength of the material. As such, the application of a low cost continuous annealing according to the invention, in combination with an electro galvanized coating, reduces the production cost differential compared to hot dip galvanizing.

The inventive BH steel microstructure has predominantly equiaxed ferrite with minor amounts of pearlite, cementite and possibly bainite and retained austenite and martensite—the latter less than 3 in volume percent. Volume fractions of martensite are kept to below 3%, since a greater amount would deteriorate BH2 values.

Copper values up to 0.5% and Ni up to 0.3% are added for weathering steels grades and/or for enhancing carbon activity and thereby resulting in higher BH values for a given total solid solution C level. Values of Al and Si up to 1% can have the same effect and thereby result in higher BH values by enhancing C activity within the steel. Finally, Ti can be utilized to slightly above stoichiometric levels and/or B up to 0.0070 can be used in order to combine with nitrogen.

A preferred furnace temperature for hot rolling of the material is Ar3+20 to 100° C. A preferred coiling temperature is greater than 650° C. for low alloy variants of less than 0.40% Mn, less than 0.1% Si and less than 0.03% P. Such a coiling temperature provides for an r-bar (average strain ratio) higher than 1.10. The preferred coiling temperature for other alloy variations is 600° C. and higher. A minimum cold rolling reduction in thickness is between 40 and 90% for r-bar values greater than 1.10. Finally, a minimum annealing temperature is 760° C., and greater than or equal to 780° C. for an r-bar value higher than 1.10. It is appreciated that a slow cooling from the annealing temperature to 720° C. ±20° C. using cooling rates between 2 and 10 K/s may or may not be applied after the annealing treatment and before rapid cooling.

In this manner, a low alloy BH steel having a minimum yield strength of 220 MPa and a yield strength as high as 380 MPa is provided without the need for an over-aging section or heating treatment. The elimination of the over-aging treatment while maintaining the excellent strength and ductility values of the material provides for a significant decrease in energy and alloy element costs.

In order to better teach the invention but not to limit its scope in any way, examples of hot rolled coils having different chemistries are provided below.

EXAMPLES

Table 1 provides the chemical compositions of eleven coils that were processed according to one or more embodiments of the present invention. As indicated in the table, Coils 1-5 had C and Mn contents that fell within Zone (A) shown in FIG. 1, Coils 6-9 had C and Mn contents that fell within Zone (B), and Coils 10 and 11 had C and Mn contents that fell within Zone (C).

Steel slabs having the chemical compositions shown in Table 1 were soaked, hot rolled and coiled. The coiled hot rolled strip was then cold rolled and annealed in a CAL before being subjected to rapid cooling down to at least 600° C. using a cooling rate that fell within the range of 20-100 K/sec. Thereafter the sheet was further cooled down to at least 140° C. using a cooling rate that fell within the range of 3-20 K/sec. The cold rolled and annealed sheet was then re-coiled and was not subjected to an overaging treatment.

TABLE 1 Coil# % C % Mn % P % S % Si % Al % N % Ti % B Others Zone (A) 1 0.0630 0.229 0.0139 0.0086 0.0070 0.0316 0.0040 0.0005 0.0026 tramp 2 0.0622 0.248 0.0112 0.0040 0.0040 0.0343 0.0023 0.0004 0.0027 tramp 3 0.0600 0.664 0.0114 0.0044 0.0040 0.0354 0.0039 0.0180 0.0001 tramp 4 0.0621 0.661 0.0139 0.0047 0.0030 0.0361 0.0043 0.0150 0.0002 tramp 6 0.0618 0.663 0.0138 0.0044 0.0033 0.0361 0.0043 0.0150 0.0002 tramp Zone (B) 6 0.0490 0.2550 0.0134 0.0055 0.0050 0.0388 0.0033 0.0140 0.0001 tramp 7 0.0407 0.2630 0.0133 0.0057 0.0080 0.0435 0.0028 0.0110 0.0001 tramp 8 0.0440 0.2950 0.0131 0.0074 0.0070 0.0307 0.0028 0.0100 0.0002 tramp 9 0.0440 0.2950 0.0131 0.0074 0.0070 0.0307 0.0028 0.0100 0.0001 tramp Zone (C) 10 0.0550 1.2400 0.0185 0.0007 0.0200 0.0446 0.0049 0.0190 0.0001 tramp 11 0.0620 1.2600 0.0233 0.0044 0.0220 0.0343 0.0039 0.0180 0.0001 tramp

The mechanical properties of the coils were measured by cutting or stamping samples or specimens from different locations on each coil and subjecting the samples to standardized mechanical property testing. Table 2 shown below provides the mechanical property data exhibited by each of the coils. In particular, Table 2 provides data for 0.2% yield strength (YS_(—)02); lower yield strength (LYS), ultimate tensile strength (UTS), percent elongation to failure (% Elong), n-value (N_val), r-value (R val), r-bar (R-bar) and BH2 values for each of the coils. It is appreciated that from the results that Coils 1 and 2 with B added, and Coils 3-5 with Ti added, exhibited BH2 values between 25-45 MPa, Coils 6-9 exhibited BH2 values between 45-60 MPa and Coils 10 and 11 exhibited BH2 values less than 25 MPa. These results, i.e. Zones (A), (B) and (C), are also indicated in FIG. 1. It is appreciated that in some instances BH2 values between 45-60 MPa are desirable, e.g. for unexposed parts/components where Lüders strain or bands is not detrimental to a required surface finish.

TABLE 1 YS_G2 LYS UTS Coil# (MPa) (MPa) (MPa) % Elong. N-value R(0)-value R_bar BH₂ 1 261.6 261.5 388.6 36.5 0.170 1.19 1.231 32.5 2 230.2 230.2 356.8 40.0 0.193 1.14 1.115 36.5 3 306.6 303.5 408.9 36.5 0.185 1.05 1.120 34.8 4 300.6 302.7 412.1 36.0 0.174 1.14 1.200 32.9 5 302.7 306.0 412.6 35.6 0.172 1.14 1.230 30.3 6 273.5 273.3 374.9 35.5 0.18 1.13 1.1 50.0 7 281.3 281.3 383.5 35.9 0.18 1.23 1.19 53.7 8 251.4 251.4 371.9 37.2 0.191 1.36 1.17 51.1 9 272.1 272.1 377.4 36.2 0.188 1.17 1.14 51.0 10 362.7 359.1 455 31.4 0.174 1.13 1.04 21.6 11 391.8 385 465 32.8 0.179 1.13 1.02 20.6

Mechanical property testing and data were also taken at different locations along the length of a given coil. In particular, samples were taken and tested from the head (H), middle (M) and tail (T) of the coils. The results of this testing are shown in FIGS. 5-11 where the triangle data symbols represent 220-300 MPa yield strength grade alloys, the circle data symbols represent 280-340 MPa yield strength grade alloys and the square data symbols represent 300-380 MPa yield strength grade alloys. Also, and as observed in the figures, relatively uniform, and in some cases excellent uniform, mechanical properties were provided by the inventive steels.

Regarding microstructures of the inventive steel alloys, FIGS. 12-17 provide optical micrographs taken at: (A) 100×; (B) 500×; and (C) 1000× for microstructures etched with 2% nital. In addition, FIGS. 12 and 13 show the longitudinal direction and transverse direction microstructures, respectively, for a 220-300 MPa yield strength grade alloy. FIGS. 14 and 15 show the longitudinal direction and transverse direction microstructures, respectively, for a 280-340 MPa yield strength grade alloy. And FIGS. 16 and 17 show the longitudinal direction and transverse direction microstructures, respectively, for a 300-380 MPa yield strength grade alloy. The grains size for all of the samples was ASTM 11.0 or finer/smaller.

The scope of the invention is not defined by the examples and embodiments disclosed above. Changes, modifications, and the like of the composition and thermomechanical treatment of the material that fall within the scope of the invention will be obvious to one skilled in the art. As such, the scope of the invention is defined by the claims and all equivalents thereof. 

We claim:
 1. A process for producing high strength bake hardenable low alloy steel without the use of an overaging treatment, the process comprising: providing a steel slab having a chemical composition in weight percent within a range of 0.020-0.080 C, 0.20-1.60 Mn, 0.0-1.50 Si, 0.0-0.08 P, 0.0-0.60 Cr, 0.0-0.40 Mo, 0.0-0.50 Cu, 0.0-0.30 Ni, 0.04-0.5 Al and balance being Fe and incidental impurities; soaking the steel slab; hot rolling the steel slab and producing hot rolled strip; coiling the hot rolled strip using a coiling temperature of at least 600° C.; cold rolling and annealing the coiled hot rolled strip in a continuous annealing line at a temperature equal to or greater than 760° C.; and cooling the annealed cold rolled sheet using a rapid cooling rate greater than 20 K/sec down to at least 600° C., the annealed and cooled cold rolled sheet having a minimum yield strength of 220 MPa and a BH2 value between 20 and 60 MPa.
 2. The process of claim 1, wherein the cooling rate is between 20 and 100 K/sec.
 3. The process of claim 1, wherein the annealed cold rolled sheet is cooled to at least 375° C. using a cooling rate between 50-120 K/sec.
 4. The process of claim 3, wherein the annealed and cooled cold rolled sheet spends less than 4 minutes in a cooling section of the continuous annealing line.
 5. The process of claim 1, wherein the BH2 value of the annealed cold rolled sheet is between 25 and 60 MPa.
 6. The process of claim 5, wherein the BH2 value is between 25 and 45 MPa, and C and Mn contents of the steel slab obey the relation: 25≦−1300·C+118−{(−1300·C+118)·0.5·(Mn−0.2)}≦45   (1)
 7. The process of claim 1, further including cooling the annealed cold rolled sheet using a slow cooling rate between 2-20 K/sec down to 720° C. ±20° C. before cooling the annealed cold rolled sheet using the first cooling rate.
 8. The process of claim 1, further including cooling the annealed cold rolled sheet using a less rapid cooling rate between 3-20 K/sec to a temperature of at least 140° C. after the annealed cold rolled sheet is cooled down to at least 600° C.
 9. The process of claim 1, wherein the Mn content is less than 0.40, the Si content is less than 0.1, the P content is less than 0.03 and the hot rolled strip is coiled at a temperature equal to or greater than 650° C.
 10. The process of claim 1, wherein the coiled hot rolled strip is annealed at a temperature equal to or greater than 780° C.
 11. The process of claim 1, further including hot dip coating the cooled cold rolled sheet, the hot dip coated cold rolled sheet having the minimum yield strength of 220 MPa and the BH2 value between 20 and 60 MPa.
 12. The process of claim 11, wherein the hot dip coating is hot dip galvanizing.
 13. The process of claim 12, wherein the steel slab has less than 0.04 C and the C and Mn contents obey the relation: 25≦−1300·C+118−{(−1300·C+118)·0.5·(Mn−0.2)}≦45   (1)
 14. A bake hardenable low alloy cold rolled and annealed steel sheet produced without the use of an overaging treatment, said steel sheet comprising: a chemical composition in weight percent within a range of 0.020-0.080 C, 0.20-1.60 Mn, 0.0-1.50 Si, 0.0-0.08 P, 0.0-0.60 Cr, 0.0-0.40 Mo, 0.0-0.50 Cu, 0.0-0.30 Ni, 0.04 -0.5 Al and balance being Fe and incidental impurities; and a minimum yield strength of 220 MPa and a BH2 value between 20 and 60 MPa.
 15. The steel sheet of claim 14, wherein said BH2 value is between 25 and 60 MPa.
 16. The steel sheet of claim 15, wherein said BH2 value is between 25 and 45 MPa and said C and Mn contents obey the relation: 25≦−1300·C+118−{(−1300·C+118)·0.5·(Mn−0.2)}≦45   (1)
 17. The steel sheet of claim 16, wherein said Mn content is less than 0.40, said Si content is less than 0.1 and said P content is less than 0.03.
 18. The steel sheet of claim 14, wherein said Mn content of between 0.8 and 1.4.
 19. The steel sheet of claim 14, further comprising said chemical composition having at least one of a Ti content up to 0.025 and a B content up to 0.0070. 